Non-impact recording methods have attracted favorable attention as a means for making a hard copy of electronic picture information because they make less noise in recording. An ink jet recording method in which regular paper can be used for recording and in which recording can be accomplished without carrying out any special processes, such as photographic fixing, has been regarded as a very useful recording method.
In a conventional ink jet recording method, a pressure pulse is applied to an ink containing member during recording to jet ink from an ink-jet opening or orifice of the member. It has, however, been difficult to build a small ink jet device to implement the conventional ink jet printing method. Also, in order to perform printing with acceptable density, mechanical scanning has been required for the ink jet device. Consequently, the speed of the conventional ink jet method has been greatly limited.
Recently, several techniques, such as a magnetic ink jet method, a plane ink jet method, and a thermal bubble ink jet method, have been proposed to eliminate the aforementioned problems in order to make high-speed ink jet printing possible. In the magnetic ink jet method, a magnetic field is applied to magnetic ink provided in the vicinity of a magnetic electrode array to produce a meniscus in the surface of the magnetic ink and to record with a given pel density. An electrostatic field is applied to the magnetic ink to jet droplets of the magnetic ink. The magnetic ink jet method, however, has a disadvantage in that variable color imaging becomes difficult because of the coloration of magnetic powder contained in the ink.
In the plane ink jet method, ink disposed in a slit-like ink reservoir parallel to an electrode array is caused to jet in accordance with an electric field pattern formed between the electrode array and an array of opposite electrode. A recording paper is interposed between the two arrays. Although the plane ink jet method has an advantage in that a small orifice is not required and, therefore, the problem of orifice blockage due to the drying of ink in the orifice is avoided. The method has a disadvantage, however, in that a high voltage is required for causing the ink to jet. It is necessary to use time-division driving of the electrode array in order to prevent voltage leakage between adjacent electrodes. As a result, the plane ink jet method is not suitable for high-speed ink jetting.
In the thermal bubble jet method, ink is subject to thermal energy and is rapidly heated to produce surface boiling. Consequently, bubbles are formed within an orifice to jet the ink due to the increase of pressure within the orifice. In the thermal bubble jet method, it is necessary to raise the temperature of an exothermic material rapidly to cause surface boiling. Accordingly, the method has a practical disadvantage in that thermal transmutation of ink and thermal degradation of a layer for protecting an exothermic resistor provided as a heating means often occur.
In order to improve the recording speed of conventional ink jet methods and to avoid the disadvantages of the aforementioned ink jet methods, a so-called thermal electrostatic ink jet method has been proposed. In this method, a thermal signal is applied to the ink, and simultaneously or successively an electrostatic field is applied to the ink to jet the ink at the heated locations thereof.
The thermal electrostatic ink jet recording head used in the thermal electrostatic ink jet method comprises exothermic materials (or exothermic elements) for applying thermal energy to ink, an electrostatic induction electrode electrically connected to the ink to apply electrostatic energy to the ink, and means for feeding and holding the ink to and at an ink orifice to facilitate jetting of the ink due to the electrostatic power.
Specifically, the recording head includes a slit-like space formed between a first plate member formed on an insulating substrate and a second plate member. The first plate member may be formed by an exothermic resistor array composed of a plurality of exothermic resistors. The second plate member is disposed opposite to the first plate member and is separated therefrom by slit-like space having a predetermined width. The ink is fed into the slit-like space and held by pressure members, such as a pump, or the like to feed and hold ink within the slit-like space. In the head, the electrostatic induction electrode is provided on one of the first and second plate members.
The present inventors have found that when a voltage is applied across an electrically conductive layer or electrode for inducing an electrostatic field and an electrically-conductive layer or counter electrode with a recording medium interposed therebetween, gas discharge occurs between the electrostatic field induction electrode and the counter electrode at ink free locations on the ink-jet side end portion of the head formed by the two plate members. The gas discharge phenomenon causes problems in that stable ink jetting is prevented and safe operation is more difficult.